Topics Topics Edit Profile Profile Help/Instructions Help Member List Member List  
Search Last 1|3|7 Days Search Search Tree View Tree View  

Spare op amp amplifiers

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 29 November, 2010 » Spare op amp amplifiers « Previous Next »

  Thread Last Poster Posts Pages Last Post
  ClosedClosed: New threads not accepted on this page        

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

davel
Regular Contributor
Username: davel

Post Number: 39
Registered: 11-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Saturday, 27 November, 2010 - 11:15 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,
If there is an unused op amp in a design, say using 3 of the four op amps on a chip is it best to leave them floating or tie them to the ground or postive rails.I have seen designs were they are floating taken to ground directly and via a 1k resistor.Which is best and why ?
Thank you.
Dave Larner
Top of pagePrevious messageNext messageBottom of page Link to this message

mikehibbett
Frequent Contributor
Username: mikehibbett

Post Number: 1144
Registered: 04-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Saturday, 27 November, 2010 - 11:21 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Never leave any input floating unless you know it is tied high or low internally.

Mike
Top of pagePrevious messageNext messageBottom of page Link to this message

gordon
Frequent Contributor
Username: gordon

Post Number: 640
Registered: 05-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Saturday, 27 November, 2010 - 03:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

There is some info. here. Seems floating inputs could cause oscillation, or wasted power.

http://www.analog.com/static/imported-files/rarely_asked_questions/RAQ_unused_op-amp.pdf
Top of pagePrevious messageNext messageBottom of page Link to this message

steerpike
Frequent Contributor
Username: steerpike

Post Number: 522
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Sunday, 28 November, 2010 - 02:07 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Tying an op-amps two inputs together, or to rails can also upset it.

My preference is to tie the unused output to the unused '-' input (make it a buffer) and the '+' input to another op amps + or - input, so the unused one follows a used one's signal.
Top of pagePrevious messageNext messageBottom of page Link to this message

davel
Regular Contributor
Username: davel

Post Number: 40
Registered: 11-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Sunday, 28 November, 2010 - 10:15 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for your replies which were much appreciated, I've seached though a few op amp books and none covered this issue. The article from Gordon was very interesting and I Have incorpoated the spare as a buffer although its not needed ! Problem solved.
Dave.
Top of pagePrevious messageNext messageBottom of page Link to this message

joe
:::: Super User ::::
Username: joe

Post Number: 997
Registered: 05-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Sunday, 28 November, 2010 - 10:59 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

This is a snip from the Texas Instruments Opamp guide that I grabbed off their website many moons ago.

If you want a copy of the PDF (it's 2Mb), drop me an Email.


Read my ramblings - www.techbites.com/joe-farr
Top of pagePrevious messageNext messageBottom of page Link to this message

atferrari
Frequent Contributor
Username: atferrari

Post Number: 976
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Sunday, 28 November, 2010 - 07:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola Joe,

Just in case, give a check. Some moons later, a new wizard of the tribe revamped parts of the original (Mancini's IIRC) including new technologies.
Agustín Tomás - Buenos Aires - Argentina

Administration Administration Log Out Log Out   Previous Page Previous Page Next Page Next Page